Wave translation



THERMI$TOR vvvv H. K. KRIST WAVE TRANSLATION Filed Feb. 25, 1941 l3 MAMLAT SLCWE April 28, 1942., 1

FIG. .5

/NVEN7'0R I H K KRIS? ATTORNEY Patented Apr. 28, 1942 UNi 2,281,312 IWAVE TRANSLATION Henry K. Krist, Mountain Lakes, N. Y.,

to Bell Telephone Laboratories,

assignor Incorporated,

New York, N. Y., a corporation of New York Application February 25,1941, Serial No. 380,423

\ 7 Claims.

This invention relates to electric wave amplification and moreparticularly to electric wave amplifiers of the stabilized feedbacktype.

Heretofore in long distance signaling systems comprising signalrepeaters spaced apart in a long transmission line, a pilot wave offixed initial intensity has been transmitted at low level through thesystem concurrently with the signals and employed at the severalrepeaters for variably controlling the amplification or gain thereof tocompensate for variations in the attenuation of the line and similareffects. In systems of this character the pilot wave is ordinarilydiverted in part at the output of each repeater and separately amplifiedto an intensity sufiicient to operate a repeater gain controlling deviceor the like. It is highly desirable that the diversion be so effectedthat the over-all gain of the repeater is substantially the same at thepilot wave frequency as at the signal frequencies, so that throughoutthe system the pilot wave accurately reflects the variations intransmission equivalent experienced by the signals.

A principal object of the present invention is to provide an improvedrepeater amplifier adapted for a system of the kind described and moreparticularly one of such character that the diverted pilot wave isamplified in relatively great degree within the repeater amplifier,enough so in fact that separate amplification of the diverted pilot maybe omitted. Another and more general object is to provide an improvedstabilized feedback amplifier adapted for the concurrent amplificationand through transmission of signals and waves of non-signal frequencywith partial diversion of the waves of non-signal frequency andrelatively great amplification thereof Whereas it has been proposedheretofore, as I disclosed in United States Patent 2,15,888 to H. S.Black dated April 18, 1939, to derive pilot current by applying afrequency selective shunt to the mu circuit of a stabilized feedbackrepeater amplifier, thereby reducing by feedback the effect of theshunting circuit on the over-all gain characteristics of the repeater,the present invention in one aspect involves appreciation andutilization of the fact that high loss at the pilot frequency may be sointerposed in the mu circuit,

either incidental to bridging the mu circuit or otherwise, that there istremendously increased amplification of the pilot current withoutdeleterious effect on the undistorted load carrying capacity or othersignificant characteristics of the amplifier.

In accordance with a feature of the invention the mu-beta or feedbackloop of an amplifier is provided with electrically spaced outputconnections adapted respectively for signal. and pilot frequencies orfor other respectively differentfrequencie's. In another aspect of theinvention, a signal amplifier having a. mu-beta loop is provided with apilot current or like output circuit so connected that a portion of theloop that is effectively in the mu circuit for signal amplification,lies effectively in gain increasing relation in the beta circuit forpilot current amplification.

Another feature of the invention provides for transfer of the power loadat the pilot frequency from the signal output stage to an anterior stagewhereby the power load on a plurality of stages is more nearly equalizedor otherwise more fav orably apportioned.

The nature of the present invention and its various objects, featuresand advantages will appear more fully from a consideration of thefollowing description of the embodiments illustrated in the accompanyingdrawing.

Fig.. 1 illustrates diagrammatically a signal transmission systemcomprising a signal repeater in accordance with the invention;

Fig. 2 illustrates schematically the circuit details of an amplifier inaccordance with Fig. 1;

Figs. 3 and 4 illustrate diagrammatically modifications of the Fig. 1amplifier; and

Fig. 5 illustrates in fairlyco'mplete detail a repeater amplifier inaccordance with the invention having special advantages and featuresthat will be described hereinafter.

Referring now to Fig. 1 there is shown diagrammatically a signalingsystem in which signals from a source l and currents of non-signalfrequency from a source 2 are transmitted through a wire line having atleast one repeater R disposed therein. It may be convenient to think ofthe source 8 as the transmitting terminal circuits of a multiplexcarrier telephone system delivering telephone signals occupying aplurality of carrier channels- Likewise, it is thought that theprinciples underlying the invention may be more readily grasped if it besupposed that source 2 delivers a single-frequency pilot wave that liesabove, below or between the telephone channels. It will become apparent,however, that the invention is not limited in its applicationto theparticular case assumed and that there may be, for example, a pluralityof pilot frequencies or that the frequency may be used to supply carriercurrent for demodulation at a receiving terminal,

diverted non-signal I The negative feedback repeater amplifierillustrated in Fig. 1 comprises a mu-beta loop having an input terminala for they application of signal and pilot waves from the incoming line,

where ;t represents the transmission equivalent of the mu portion of theloop and 5 the transmission equivalent of the beta portion. Assumingthat at all frequencies of interest the vectorial product y-fl is largecompared with unity,

the gain can be represented as The normal gain characteristic of theamplifier is therefore seen to be dependent substantially only on thecharacteristics of the beta circuit network 5, and the latter may bereadily designed to provide constant gain over the frequency range ornon-uniform gain proportioned, for example, to compensate for theattenuation frequency characteristics of the transmission line.

Consider now that in the mu circuit at a point p between amplifiersection 3 and output terminal s some frequency selective means isprovided for diverting current of pilot frequency. In so far as thediverted pilot current is concerned the point 12 may be considered theoutput terminal of the amplifier, so that the mu circuit thereforconsists of amplifier element 3, and the corresponding beta circuit nowincludes both networks 4 and 5. With respect to the pilot currentdiverted at point p, the net amplifier gain is measured by orproportional to whatever total loss is interposed in the beta circuit,that is, the loss at the pilot frequency, associated with the pilotdiverting means and networks A and 5.

At the same time the over-all amplifier gain for through transmission ofpilot and signals from a to s is substantially equal to the loss ofnetwork 5, which should be so proportioned that pilot and signals areamplified to substantially the same degree.

Circuit details of a simple form of repeater amplifier in accordancewith Fig.- 1 are shown schematically in Fig. 2. The normal input andoutput connections to the amplifier are made by transformers 8 and 9,respectively. In the mu circuit of the amplifier a single amplifyingdischarge device I3 is shown, although it will be apparent that it couldbe replaced by a plurality of stages in tandem. The normal beta circuitof the amplifier comprises a connection from a tap on the primary of theoutput transformer 9 to the secondary of input transformer 8, resistanceI5 constituting the coupling impedance. Across the output terminals ofamplifying device l3 there is disposed a shunt branch 20 comprisingcapacitance l0 and inductance ll connected in series with each other andproportioned for resonance at the pilot frequency. The pilot outputcircuit P comprises a coil l4 inductively coupled. with inductance II,and the effective resistance of this circuit is representedschematically by a resistance l2 in series in the shunt branch 20.

In series between the shunt branch last described and output transformer9 there is interposed a network 30 comprising an inductance l6 andresistance l1 disposed in series with each other and shunted by acapacitance Id. Inductance l0 and capacitance 18 are adjusted foranti-resonance at the pilot frequency, and the impedance presented andthe loss introduced thereby are controlled by the magnitudes ofresistance II and of reactance It.

Now it will be seen that a certain amount of loss at the pilot frequencyis interposed in the normal mu circuit at the point where the pilot isdiverted, by virtue of the effective resistance of the pilot shunt 20and associated output circuit, and that an indefinitely great amount ofloss may be interposed immediately following this point by the seriesbranch 30. Both of these losses areeffective in reducing the amount offeedback at the pilot frequency, with respect to the diversion point pof Fig. l, and both therefore contribute in proportion to theirmagnitudes to increased effective gain at the pilot frequency from inputpoint a to output point With ts large compared with unity the increasein gain so obtained is approximately equal to the loss introduced.

Whereas the mu circuit loss at the pilot frequency is provided in Fig. 2by both the shunt branch 20 and by the series network 30, and either onealone would serve to some extent for that purpose, the combination ofboth together has other advantages and is especially useful wherecontrol of the circuit impedances is de sired as will presently appear.

For maximum power output from an amplifier tube, the tube must work intothe correct impedance, which depends on the type of tube and on theassociated circuit conditions. In a typical example involving a pentodeamplifier tube of 35,000 ohms plate impedance, the optimum loadimpedance was found to be about 3,000 ohms. With departure from theoptimum load impedance, the power which it is possible to deliver to aload, with maximum grid voltage swing, is reduced due to currentlimitation or to voltage limitation of the particular vacuum tubecircuit. In Fig. 2 output transformer 9 is accordingly so proportionedas to present the optimum load impedance, of 3,000 ohms for example, topentode l3 throughout the signal frequency range.

If either network 20 or network 30 is alone interposed between the anodeof tube [3 and output transformer 9, the impedance presented to the tubeat the pilot frequency will thereby be changed from the optimum valueavailable at output transformer 9. This change tends to reduce the totalpower diversion obtainable at the pilot frequency.- In view of the factthat the power demanded from the tube at the pilot frequency isgenerally small compared with the signal power, an even more importanteffect of departure from optimum impedance is that the load-carryingcapacity of the tube is reduced also at the signal frequency andaccordingly less power can be delivered to output transformer 9.

In other words, if a tube delivers power at two frequencies to twoseparate loads, the maximum power at one of the frequencies can beobtained only if the impedance at the other frequency is adjusted to itsoptimum value. From another of the second stage 24 thereby increasingthe point of view it may be said that the presence of the pilotfrequency changes the instantaneous operating point of the tube withrespect to the superimposed signal frequencies and that this change inoperating point should be made along the optimum load line at the pilotfrequency. Expressed in a slightly different manner, the instantaneousvoltage and current components of the frequencies may be considered asbeing additive so that each of the components should be capable ofachieving maximum value.

With the aid of networks 20 and 80 in Fig. 2, optimum impedance can bepresented t6 the pentode l3. Thus the impedance presented by the pilotdiverter circuit P to the pentode I3, and therefore also the powertransmission into that circuit, is susceptible of adjustment. If network30 is proportioned to have high series impedance at the pilotfrequency,such for specific example as an impedance one hundred times the outputimpedance of the amplifier or 300,000 ohms, it will be apparent that atthe pilot frequency the respective impedances of the shunt branch and'the output transformer are eifectivelyisolated from each other so thatthey may be independently adjusted to the most favorable values.

The output impedance at the pilot frequency presented by the amplifierto the outgoing line is normally rather uncritical. Moreover, theimpedance modifying effect of networks 20 and 30 is substantiallyreduced by the negative feedback existing in the amplifier at-the pilotfrequency. If in any case the residual effect were consideredobjectionable, however, it could be eliminated by the use of anadditional shunt branch 40, similar to shunt branch and interposedbetween network and output transformer 9. I

In a particular case in practice substantially conforming with thetypical example described 1 with reference to Fig. 2, the virtualresistance I? of the pilot output circuit P was adjusted to 3,000 ohmswhereby the shunt branch in conjunction with network 30 introduced aloss of 14 decibels into the feedback path. In this case the powerdelivered tothe pilot output circuit P was more than twenty times thepilot power delivered to the outgoing line, although this is by I nomeans the limiting value of power multiplicaffreq uency characteristicof network 30 which is :effective in relatively reducing the amount offeedback at the pilot frequency.

Whereas in Fig. l the pilot power is indicated .as being diverted at apoint following the last stage of the amplifier element 3', advantagesmay be secured in particular cases by diverting the pilot power at apoint anterior to the last stage. In an amplifier such as represented inFig. 3 comprising three amplifying stages 23, 24,- 25, the last stagemay normally operate nearer to overload than the second stage. If insuch case pilot power is to be diverted from the amplifier in accordancewith the invention, the pilot diverter circuit P may be connected at theoutput power load on that stage and providing increased power margin inthe last stage. Although the diverted pilot power may be less when theconnection is made anterior to the last stage, there may bemoreeffective utilization of the power capacity of the several stages.

If, with respect to the normal amplifier 11-8 in Fig. 1, p is not solarge that the loss at the pilot frequency introduced in the mu circuitcan be ignored, then the gain-frequency characteristic of the normalamplifier may exhibit an abrupt variation in gain at the pilotfrequency. This effect may be reduced or substantially eliminated byproviding increased gain in the mu circuit of the normal amplifier. Theincreased gain at the pilot frequency may be obtained as indicated inFig. 3 by providing positive feedback at the pilot frequency around oneor more of the local stages. With the arrangement shown the increasedgain is effective not only in the normal amplifier but also in the mucircuit of the pilot amplifier portion ap.

It maynot be necessary in all cases to operate the discharge device l3into its optimum impedance at the pilot frequency, as for example wherethe maximum pilot power to be diverted through the shunt branch is verysmall compared to the load carrying capacity of the discharge device. Insuch cases the series network 30 can be omitted and the value of virtualresistance l2 reduced until the amount of power diversion for the pilotfrequency is obtained. This can be readily accomplished inasmuch as thevoltage across resistance I2 is maintained substantially independent ofthe magnitude of resistance l2 by virtue of the constant voltage type'of feedback indicated in Fig. 2. Alternatively, the shunt branch can beomitted and the pilot output circuit P coupled to the series network 30,as by inductive coupling to the inductance l6 thereof. The first ofthese alternatives is illustrated in Fig. 5 and the second in Fig. 4which will now be described.

The circuit shown schematically in Fig. 4 is essentially of the same,configuration as Fig. 2

except that the shunt branch is omitted and the pilot output circuit Pis inductively coupled by coil Id to the inductance I6 comprisingnetwork 30. The effective resistance in the network, comprisingthevirtual resistance of the pilot output circuit P, is represented byresistance [1. Element I! might in fact furnish the pilot frequency loaddirectly without the coupling M. The gain from the input of theamplifier to the pilot output circuit is enhanced by the loss at thepilot frequency in the mu circuit of the normal amplifier, introduced bynetwork 30 and by the pilot output circuit? connected thereto. As inpreceding examples, the diverted pilot power may be many times as greatas the pilot power delivered with the signals to the outgoing line.

"In Fig. 5 applicant has shown in considerable detail a practicalrepeater amplifier illustrating one embodiment of the invention. Theampli-- fier proper is preceded by, an equalizer 3| which compensatesfor the nonuniform attenuationfrequency characteristic of the precedingline section, and by flat gain and slope control circuits 32 and 33,respectively. The latter are controlled by diverted pilot power in amanner and for a purpose to be described. The amplifier comprises threestages impedance-coupled in tandem relation and adapted to amplify withsubstantially constant gain over a frequency range extending from 15 to30 kilocycles, for example. The cathodes of the several amplifierpentodes A I, 42 and 63, are connected through respective grid biasingresistor-condenser combinations to one corner w of an output circuitbridge of which an opposite corner 1! is grounded and connected througha lead 45 to provide negative feedback to the first stage of theamplifier. The remaining corners a: and z of the bridge are connected tothe respective primary terminals of output transformer M, and corner a:is connected also to the anode of pentode t3.

In a particular case in practice where pentode Ali was of the WesternElectric 311-A type and where output transformer M presented animpedance of 4,000 ohms across its primary winding, the resistor iconnecting bridge corners w and a was 100 ohms, resistor 52 connectingcorners y and a was 1,140 ohms and the, resistor 53 connectingdiagonally opposite corners w and 1 was 32 ohms. In lieu of a singleresistor conof undesirably high resistance, three resistors arranged inthe form of a T are employed instead. Two of these resistors tit and t5are connected in series with each other between points :1: and y, and inthe example mentioned the first was 44,400 and the second 824 ohms. Thethird resistor 56 is connected between the junction 22 of the first tworesistors and corner w of the bridge, and in the same example was 103ohms.

The pilot diverting circuit in Fig. 5 comprises a capacitance i8 andinductance H, connected in series with each other across bridge points wand :c, and respectively corresponding with the the like designatedelements of Fig. 2. Coil i l coupled to inductance ll supplies pilotpower to tively associated with thermistors 5i and 52.

Thermistor Si is connected to or constitutes an element of network 3|which is so arranged that as the resistance of the thermistor varies inresponse to changes in the pilot power in circuit 50, the loss itintroduces into the circuit is changed uniformly over the frequencyrange to maintain the pilot power in circuit 50, and also at the outputterminals of the repeater, substantially constant. Thermistor 52 issimilarly associated with network 33 and the changes in its resistanceaccompanying changes in the pilot power are translated into changes inthe slope of the attenuation-frequency characteristic of the repeater.for regulating the'fiat gain and slope characteristics of a repeaterunder the control of a pilot wave is well known in the art and does notof itself constitute a part of the present invention.

Bearing in mind that the pilot output power and the diverted pilot powerboth vary over only a very slight range when the regulating system isproperly adjusted, it would evidently be desirable if these slightchange could be translated into magnified changes in heating currentsupplied to the thermistors 5| and 52 so that a correspondingly widerange of resistance values could be utilized. This desirable object issecured in Fig. 5 by interposing in the pilot circuit 50 one or moredirectly heated thermistors 51, 58. The thermistor are so proportionedand so connected in the circuit that as the pilot current from coil Hchanges they change in resistance in such sense as to amplify the changein current appearing at thermistor heaters 53 and 54.

The use of thermistors as described Thus, if a single thermistor havinga negative temperature coeificient of resistance is disposed in seriesin circuit 50 an increase in the pilot voltage at coil M will cause amore than proportionate increase in the current flow inasmuch as with anincrease in current the thermistor is heated, its temperature raised andits resistance lowered, thereby reducing the total amount of resistancein the circuit connected to coil l4. Where two such series thermistorsare employed as in Fig. 5, a resistance 59 may be bridged across thecircuit 50 from their junction so that the expanding or amplifyingeffect of the first thermistor is in turn augmented by the expandingeffect of the second thermistor. Thermistors El, 52, 51 and 58 may beprovided with supplemental heating means to compensate for the eiiect ofvarying ambient temperature on their operating characteristics. V

If desired, positive feedback may be provided at the pilot frequency tocompensate for the efiect described with reference to Fig. 3 and forthis purpose a frequency selective network 26 may be connected as shownfrom the anode to the control grid of the second stage amplifier tube4!.

Although the present invention has been described with reference tocertain specific embodiments and with emphasis on its application topilot channel control of the transmission characteristics of a repeater,it will be evident to those skilled in the art that the invention issusceptible of various other embodiments and applications within thespirit and scope of the appended claims.

What is claimed is:

1. In a system for the pilot channel control of repeater gain, arepeater amplifier of the negative feedback type, means for applyingsignal and pilot currents to said amplifier for through transmissionwith substantially equal gain, means at a point in the mu circuit ofsaid amplifier for diverting pilot current therefrom, and means in saidmu circuit following said point introducing 'many times greater relativetransmission loss for said pilot current than for said signal current.

2. An electric wave amplifier of the stabilized feedback type comprisinga mu-beta loop, means for applying signals and currents of non-signalfrequency to the input of said amplifier for amplification therein,output means for withdrawing concurrently the amplified signals andnonrepeat'er gain, a repeater amplifier of the negative feedback type,means for applying signa and pilot currents to said amplifier forthrough transmission with substantially equal gain, means at a point inthe mu circuit of said amplifier for diverting pilot .current therefrom,means in said mu circuit at or. following said point introducing manytimes greater relative transmission loss for said pilot current than forsaid signal current, said diverting means comprising a tuned circuitshunted across said mu circuit and a load circuit connected theretohaving an effective resistance at the frequency of said pilot currentsuch that the greater part of which said pilot diverting means comprisesa frequency selective circuit disposed inshunt across said mu circuitandin which said amplifier is of the constant voltage feedback tym. 5.An electric wave amplifier, means for applying signal and alternatingnon-signal currents to the input of said amplifier, means forconcurrently withdrawing the amplified signal and non-signal currentsfrom the output, of said amplifier, said amplifier being of the negativefeedback type having e mu circuit and e beta circuit, means at a pointin said mu circuit for selectively diver 1- said non-signal currents,the transmission equivalent of the portion oi said mu circuit ieilowsaid point being such that said noneurrente are highly attenuatedtherein z type havinn'a mu ta loop, means for meted currents.

follcg said diverting mmns, said last-mentioned element inter-posingrelatively high loss at the frequency of said non-signal currents andrelatively low loss at the frequency or said signal currents.

E. An amplifier or the negative feedback ascomprising a mu-beta loop, mefer applyin message current and modulated altetmg currents to the inputoieeid aplmer, a frequency selective load circuit lacross eaid mucircuit and tuned to admit said cui'uiaied currents, and frequencyselective posed in said mu circuit $011 'w 1 .1, 381d cuit end tuned torelativelyime said .i

